The present invention relates to a guide for directing a thimble into a fuel assembly in a nuclear power plant, and more particularly to a thimble guide whose length can readily be changed.
A typical pressurized water reactor includes a reactor vessel which contains nuclear fuel, a coolant (water) which is heated by the nuclear fuel, and means for monitoring and controlling the nuclear reaction. The reactor vessel is cylindrical, and is provided with a hemispherical bottom and a hemispherical top which is removable. Hot water is conveyed from and returned to the vessel by a reactor coolant system which includes one or more reactor coolant loops (usually as three or four loops, depending upon the power-generating capacity of the reactor). Each loop includes a pipeline to convey hot water from the reactor vessel to a steam generator, a pipeline to convey the water from the steam generator back to the reactor vessel, and a pump. The steam generator is essentially a heat exchanger which transfers heat from the reactor coolant system to water from a source that is isolated from the reactor coolant system; the resulting steam is conveyed to a turbine to generate electricity. During operation of the reactor, the water within the vessel and the coolant system is maintained at a high pressure to keep it from boiling as it is heated by the nuclear fuel.
Nuclear fuel is supplied to the reactor in the form of a number of fuel assemblies. Each fuel assembly includes a base element called a bottom nozzle and a bundle of fuel rods and tubular guides which are supported on the bottom nozzle. The fuel rods have cylindrical housings which are filled with pellets of fissionable material enriched with U-235. The tubular guides accommodate measuring instruments and movably mounted control rods of neutron-moderating material. A typical fuel assembly for a pressurized water reactor is about 4.1 meters long, about 19.7 centimeters wide, and has a mass of about 585 kg, and a typical four loop reactor might contain 196 such fuel assemblies supported parallel to one another on a core plate within the reactor vessel. After a service life during which the U-235 enrichment of the fuel assemblies is depleted, the reactor is shut down, the pressure within the vessel is relieved, the hemispherical upper cap of the vessel is removed, and the old fuel assemblies are replaced by new ones.
A number of measuring instruments are employed to promote safety and to permit proper control of the nuclear reaction. Among other measurements, a neutron flux map is generated periodically, such as every 28 days, using data gathered by neutron flex detectors which are moved through a number of randomly selected fuel assemblies. To guide the flux detectors during their periodic journeys, closed stainless steel tubes known as flux thimbles extend through the bottom of the reactor vessel and into the fuel assemblies which have been selected as measuring sites. This will be explained in more detail with reference to FIG. 1.
In FIG. 1, a core plate 10 that is 17.5 inches (44.5 cm) thick is horizontally mounted within a reactor vessel having wall 12, the portion of wall 12 which is illustrated being at the hemispherical bottom end cap of the vessel. A number of fuel assemblies, including fuel assembly 14, are supported in an orderly array on plate 10. Fuel assembly 14 includes a bottom nozzle 16 having four legs 18 which are joined to a platform portion 20 with a centrally disposed aperture 22 in it. For purposes of the present application aperture 22 will be deemed to be located in the plane of the lower surface of plateform portion 20. A number of fuel rods 23 are bundled together and supported on platform portion 20. Within this bundle is an instrumentation tube 24 which is aligned with aperture 22 and which extends to the top nozzle (not illustrated) of fuel assembly 14.
A bore 26 having a threaded region 28 extends through core plate 10 in alignment with aperture 22. A conventional thimble guide 30 is provided with a threaded portion and with a recessed wrench-engaging region 32 which permits technicians to screw guide 30 into threaded region 28 of plate 10 during fabrication of the reactor. After guide 30 is attached in this way, welds 34 are added for additional security. Typically guide 30 is 3.38 inches (8.58 cm) high, from the upper surface of plate 10 to the upper lip 35 of guide 30, and there is a gap of 1.37 inches (3.48 cm) between upper lip 35 and aperture 22.
A bore 36 extends through wall 12 of the reactor vessel in alignment with bore 26. A vessel-penetration sleeve 38 having an outer diameter of about 1.5 inches (3.81 cm) extends through bore 36 and is welded at 40 to provide a seal which is resistant to high pressure. A bottom mounted instrumentation column 42 mounted on plate 10 extends between bore 26 and sleeve 38. Column 42 includes a fitting 44 which is attached to plate 10 by bolts 46, an upper pipe element 48 which is joined to fitting 44 by welds 50, and a lower pipe element 52 which is joined coaxially to element 48 at a tie plate (not illustrated). Lower pipe element 52 has an inner diameter of 2 inches (5.08 cm), so that there is a gap between sleeve 38 and element 52.
In a typical four-loop pressurized water reactor (having 196 fuel assemblies 14), 58 of the fuel assemblies 14 would be randomly selected for neutron flux monitoring. Accordingly, in such a reactor it will be apparent that there would be 58 guides 30, each communicating via a respective bore 26 and bottom mounted instrumentation column 42 with a respective vessel-penetration sleeve 38. During fabrication, sleeves 38 would be installed in the reactor vessel wall 12 and guides 30 and bottom mounted instrumentation columns 42 would be installed on core plate 10, the columns 42 being secured to one another by tie plates (not illustrated). Then the core plate 10 and attached structures would be lowered into the vessel, with the sleeves 38 fitting into elements 52. In the resulting structure, the upper ends (not illustrated) of sleeves 38 are spaced apart from the lower ends (not illustrated) of upper pipe elements 48, so that sleeves 38 are not in fluid-tight communication with bottom mounted instrumentation columns 42.
The bore 54 of upper pipe element 48 typically has a diameter of 0.468 inches (1.189 cm) and terminates in a flared region 56. The bore 58 of fitting 44 is typically 0.68 inches (1.73 cm) in diameter and has flared regions at either end. The bore 26 typically has a diameter of 0.75 inches (1.91 cm). The thing to note is that the channel provided by bores 54, 58, and 26 becomes progressively wider from upper pipe element 48, to fitting 44, to bore 26. This construction facilitates manufacture of the reactor and provides guidance for thimble 60 (to be discussed shortly) while avoiding the possibility that it might become stuck in the channel.
Thimble 60 is a long stainless steel tube which begins at a plate (known as a seal table, not illustrated) outside the reactor vessel and which has a closed end (not illustrated) that is normally disposed inside a fuel assembly. Thimble 60 slidably extends through tube 24, guide 30, bore 26, bottom mounted instrumentation column 42, and sleeve 38. A stainless steel guide tube (not illustrated) is welded to the outer end of sleeve 36, and thimble 60 extends within the guide tube to the seal table, which is typically located in a shielded position near the top of the reactor vessel. Since the interior of the reactor vessel is in fluid communication with the interior of sleeve 38, it will be apparent that the guide tube provides a pressure boundary which extends around thimble 60 from wall 12 to the seal table, where a high pressure seal (not illustrated) is provided between the inner wall of the guide tube (not illustrated) and the outer wall of thimble 60. The net result is that thimble 60 provides a low-pressure access channel into the reactor from a shielded position outside of the reactor.
A flux detector (not illustrated), about 2 inches (5 cm) long, is slidably accommodated within thimble 60 and is attached to a flexible push-pull cable (not illustrated) which extends through thimble 60 to flux-mapping equipment (not illustrated) located beyond the seal table (not illustrated). At periodic intervals, typically once every 28 days, the flux detectors are pushed to the tops of thimbles 60 and are then slowly withdrawn through the fuel assemblies 14 as flux measurements are taken at different heights to provide a neutron flux map of the interior of the reactor.
Normally thimbles 60 remain inserted in the instrumentation tubes 24 of the randomly selected fuel assemblies 14 between the periodic flux mapping operations. Thimbles 60 must be withdrawn from fuel assemblies 14, however, at intervals of 12-18 months when the reactor is shut down for refueling and fuel shuttling. During the refueling operation the nuclear reaction is terminated, the pressure within the reactor vessel is relieved, and the guide tubes (not illustrated) are unsealed from the thimbles 60 at the seal table (not illustrated). The thimbles 60 (which are somewhat flexible) are then withdrawn by a distance of about 14 feet (4.27 meters) to free them from the spent fuel assemblies 14, which are thereupon removed via remote control and replaced by fresh fuel assemblies 14. Thimbles 60 are then driven into the fresh fuel assemblies 14, the reactor vessel and seal table are sealed, and power generation begins anew.
The conventional thimble guide 30 of FIG. 1 has several shortcomings. It has been found that considerable turbulence exists during operation of a reactor in the region between the upper surface of core plate 10 and the lower surfaces of platform portions 20 of fuel assemblies 14. Guides 30 expose a significant portion of thimbles 60 to this turbulence, which vibrates thimbles 60 and increases wear to an undesirable extent. Simply increasing the length of guides 30 would be undesirable because fuel assembly designs may change, including the lengths of legs 18. Since guides 30 are permanently installed at the time the reactor is built, any particular length for guides 30 that is selected at that time might make it impossible to take advantage of future design improvements in fuel assemblies. Furthermore, it has been found that fluid flow in the gaps around thimbles 60 due to the progressively widening channels from elements 48 to fittings 44 to bores 26 is sufficient to cause vibrations which increase wear. Finally, the flared regions at upper lips 35 appear to increase turbulence.